1. Field of the Invention
The present invention relates to a semiconductor laser module that includes a semiconductor laser device and an optical module including the semiconductor laser module.
2. Description of the Related Art
An integrated semiconductor laser module has been known, for example, as a tunable light source for the dense wavelength division multiplexing (DWDM) optical communication (see, for example, Japanese Patent No. 4444368).
The integrated semiconductor laser module described in the above Patent Document is obtained by integrating a plurality of distributed feedback (DFB) semiconductor laser devices each having a different oscillation wavelength, a plurality of optical waveguides, a multimode interference (MMI) optical coupler, and a semiconductor optical amplifier (SOA) on a single substrate. In this integrated semiconductor laser module, when one of the semiconductor laser devices is driven, a laser light output from the semiconductor laser device is guided through an optical waveguide that is optically coupled to the semiconductor laser device. The laser light guided through the optical waveguide is then passed through the MMI optical coupler and output from an output port. The SOA amplifies the laser light output from the output port and outputs an amplified laser light from an output facet. This integrated semiconductor laser module is used, for example, as an optical transmitter in combination with an external optical modulator, for a long-haul optical transmission in a DWDM optical communication network system.
However, when a λ/4 phase-shifted DFB semiconductor laser device is used as the semiconductor laser device in the integrated semiconductor laser module having the above configuration, about a half of the optical output is emitted from a rear facet of the semiconductor laser device, resulting in a degradation of the efficiency and an increase of the power consumption. Furthermore, when the number of channels of the MMI optical coupler is n, the optical output of the semiconductor laser device is decreased to 1/n of its original output power at the MMI optical coupler. For this reason, with the configuration of the integrated semiconductor laser module employing the MMI optical coupler, the optical loss is considerably high at the MMI optical coupler, and the optical output of the entire module eventually depends on an output characteristic of the SOA. Therefore, even if the output of each semiconductor laser device is increased by using a DFB semiconductor laser device structure with which a high output power is obtained from a front facet that is an output side of the module, which will be described later, the optical output of the entire module is not improved as expected because there is a loss of the optical output at the MMI optical coupler. Because of these factors, it has been expected to provide an integrated semiconductor laser module with which the high efficiency and the high output power can be achieved.